A major thrust of the work in this laboratory has focused on a study of cellular ion-transport processes in the thyroid, a process critical for the mediation of signal transduction and the maintenance of both epithelial and metabolic function in thyroid tissue. To this end, a cultured rat thyroid cell line was utilized to characterize the regulation of sodium-dependent iodide entrance into the cell and subsequent exit into the follicular lumen, the locus of thyroid hormone synthesis. In an extension of these studies, molecular biological techniques have been invoked to characterize alternate ion- transport processes in this cell line. Similarly, studies on the heterologous expression of ion-transport proteins in alternate cell lines, have employed mammalian expression vectors containing cDNA inserts for an anion exchange protein and a monocarboxylate transport protein. The cellular location and expression of these proteins were identified by the use of specific antibodies. As was shown to be the case for sodium-iodide entry, the expression of proton-dependent uptake of moncarboxylic acids in the thyroid requires the participation of TSH. In contrast, both anion-exchange and sodium- dependent monocarboxylate transport are uninfluenced by TSH, or by agents that stimulate growth in thyroid cells. Principal mechanisms used by the cell to maintain a constant intracellular pH involve anion-exchange and lactate transport.